History of Model Railroad Standards
Model railroading has developed a number of standards since its beginning at about 1900. Some of the more obvious standards relate to the physical dimensions such as scale and track gauge. Other standards determine physical operating requirements such as coupler design, coupler height, wheel flange size, etc. A third category of standards specifies the electrical power requirements necessary to operate the electric motors inside the engines and the specification of remote control signaling, if any.
The advantage of a standard is that it allows all contributors to the field to specify their products in ways that will allow them to operate with existing model railroads. The disadvantage of a standard is that it can be too restrictive, not allowing new ideas and inventions to be integrated into older established model train layouts. As the pressure of new technology advances, the desire for new operating capabilities can become important enough to actually demand a change from the old standard to a new standard. This is always done with great reluctance since it often involves discarding older models that cannot be modified to work within the new standard or in can require extensive redesigns of existing layouts.
Usually, in model railroading, there is little change to the physical standards and even the ones that are introduced can often be integrated into existing layouts. For instance, a new coupler design can be added to some cars and still allow the operator to use all his other equipment by adding at least one car that has the old coupler style on one end and the new style on the other. Changes to the electrical standards are much more difficult to accomplish.
Standards for AC Powered Model Trains
Commercial electric model trains in the United States were introduced about 1896 and used AC power applied to the track with AC/DC universal motors in the engines to power the wheels. The AC power, at first, came directly from 110 v household power but later, step-down transformers were used to provide a safer 20 to 30 volts of track power. When speed control was introduced, it was first accomplished by dropping the voltage to the track with resistors such as high-wattage slide-type potentiometers or switching in banks of light bulbs to vary the series resistance. In 1906, Lionel introduced variac-type transformers that could vary the track voltage more efficiently.
Remote control signaling for AC powered trains
In 1935, the Lionel corporation introduced the idea of applying a small amount of DC superimposed on the AC track power to do simple remote control operation of a whistle sound effect in their engines. Besides the difficulty of designing a reliable and inexpensive DC source, there were also technical obstacles with the DC detector in the engine. Lionel chose to use a special relay that would ignore AC and respond only to the presence of DC. This relay required a sizable amount of DC (1.1 v to 3 v) before the relay would close. Once closed, however, it only required a small amount (300-500 mV) to stay in the closed position. Unfortunately, if 5 v or more of DC was maintained on the track, the relay would often become magnetized and stay in the closed position even after the DC signal had been removed. Also, it was difficult to provide a sustained large-voltage DC signal from available rectifiers of the time that would not over-heat at the current levels required by Lionel motors. Lionel solved these problems by designing their DC remote control technique to use a lever or button that would first make contract to a source of half-wave rectified voltage and then to make contact to a second terminal that applied the full amount of track AC with a sustained superimposed 300 to 500 mV of DC. Although the half-wave rectified signal for the first contact would decrease the power to the track by more than half, it provided a very strong DC source signal and it was applied only long enough to close the horn relay in the engine. The first contact is called "horn position one" and the second contact is called "horn position two".
The early Lionel whistle sound effect used a motor to turn a fan that produced a "woooooo" like sound in two air chambers (same principle as an organ pipe). This motor required extra current from the power supply to the track and would slow the engine down. Lionel solved this problem by adding a booster winding to their transformer that added in an extra 5 to 8 volts of AC when the whistle button was pressed.
Lionel's method of applying AC power along with DC signaling and the use of the four position reversing unit (E-unit) has been the basis of their operating electrical standard since the early 1950's. At first, Lionel did not distinguish between plus or minus DC signaling since their whistle or horn sound effect would work with either polarity. Later in the 1980's, Lionel engines were introduced that had different remote control effects when positive or negative DC was superimposed on the AC power applied to the track; e.g. the horn or whistle would operate with +DC and the bell would toggle on and off with -DC. These systems used electronic detectors in the engine that would distinguish the different DC polarities. Unfortunately, these detectors required that the DC voltage component on the track exceed the amount necessary to turn on a silicon pon junction (about 0.7 to 0.9 v). The older Lionel transformers usually only produced this amount when the transformer horn button was in horn position one where half wave DC was applied. Since this reduces the power, the engines with the new electronic detectors would slow down when the whistle effect is operating even with the benefit of the whistle boost winding on their early transformers. With the transformer in horn position 2, the horn would often shut off completely or go off intermittently.
Also, when no horn signal was applied, the electronic horns had the annoying problem of periodically going off from noise on the track. This did not happen as often with the earlier horn design since the horn relay had a built in hysteresis that required a "position 1" signal to get the horn to operate followed by a "position 2" signal to keep it closed. The new electronic detectors could have been designed to maintain this standard by having hysteresis designed into the circuit. In other words, a large DC signal would be required to turn on the detector (horn position 1 ) and only a small amount required to keep in on (horn position 2). With the extra advantage of having the horn DC signal always start with a large amount of DC, the detector circuit could be designed to be more responsive to the large "position 1" horn signals to turn on quickly and, at the same time, more immune to noise spikes of DC on the track that are below the hysteresis threshold.
Most current Lionel transformer now only have a single horn position that produces from 0.8 to 5 volts of DC offset. It would have been an advantage for Lionel designers to stay within the existing standard to provide better horn detectors. Engineers and designers could have used the two horn positions to do other effects such as having the horn change pitch from horn position one to horn position two. The standard could easily have been expanded to have the value of DC voltage control some feature in a continuous analog manner (the pitch could vary continuously from a low to a high value depending on the amount of DC, etc.). Relaxing the standard to only having one horn signal position means that some older products will not work correctly with new transformers and some new products that could have been developed with the original standard will not work with modern power supplies. If the original Lionel standard is more accepted, then most of the newer power supplies will be discarded over time; however, if the new transformer design becomes the standard, then the older equipment will be discarded or changed to work with the newer horn signals. In either case, making changes to a standard can have significant implications. Again, maintaining the standard or intelligently extending the standard is the important issue. Any changes to a standard should be an expansion rather than a restriction. It is risky to make arbitrary changes to a standard since your products may become unacceptable.
There are a number of other specifications for the original standard used by Lionel that should be mentioned here:
1) Lionel also had always used full AC sine wave on the track to power the trains. The power was changed by changing the amplitude of the applied voltage by first using voltage-dropping techniques and then later by using vadacs. Some Lionel operating cars depended on the sine-wave shape of applied power for proper operation.
2) The range of applied A.C. voltage from most Lionel transformers is from completely off to turning on abruptly at 5 volts rms and then continuously variable up to 17-22 volts rms. The minimum voltage of 5 volts was chosen because most of the universal AC/DC motors used by Lionel would not start to rotate until 5 to 8 volts. When designing electronics that will operate on-board in a model train, the 5 volt minimum voltage is very useful since most simple circuits require a minimum of 3-5 volts to operate properly.
3) Lionel's also kept to the standard of using three-rail track for most of their trains which obviated the need to switch the track connections on reverse loops, and made it easier for children to operate.
Standards for DC Powered Model Trains
Also, in the mid 1940's, it was becoming possible to manufacture DC rectifiers with sufficient capacity and a low enough cost to power model railroads. This, along with the more efficient DC permanent magnet motor, allowed the model railroad field to introduce a new standard for model railroads that use DC power instead of AC power applied to the track. With on-board DC motors wired directly through the wheels to the track, the direction of model train locomotives could now be reversed by changing the polarity of DC track voltage at the power source. This gave the operator a simple method of controlling his model engine's direction by remote control. This standard is still in use today, primarily on two-rail systems. It's biggest problem is that, unlike the Lionel standard, there is no method specified to do simple remote control signaling.
Most model trains that were operated on DC power, used two-rail track with a polarity reversal switch to solve the reverse loop track problem. The range of applied track voltage is usually from 0-12 volts and most motors used in the locomotives are permanent DC "can" type. Some new power packs for G-Gauge, DC powered trains have a range from 0-17 volts DC. The applied DC voltage can be simply full wave rectified and varied over its range by changing the amplitude. However, many DC power packs will filter the DC to remove the AC ripple and control the amount of power to the track by either varying the amplitude or changing the duty cycle of a pulse drive output or a combination of both. In any case, unlike AC powered trains, there is often less of a range of applied track voltage and there is no dependable minimum voltage to power on-board electronic circuitry.
Standards for New Model Train Control Systems
One difficulty with designing any new control system is customer acceptance. It is most important that the new system not require major modification of the layout and the engines. In particular, it is an operating and marketing advantage for any new system to have the following characteristics:
It should allow older locomotives to interact with newer engines equipped with the new system receiver units (backward compatible). The new system should not require that all engines be changed before it can be used.
It should not require major rework or modification of the engines when installing any on-board new-system product that would, in turn, decrease the value of the engine.
It should operate with existing controllers and power supplies. If a new system controller/power pack is added to extend the system capabilities, it should be easily connected to the layout and require no modifications of the track.
Any on-board system product added to an engine should be usable on other layouts that use standard traditional control methods.
The new system should be flexible. The design should have enough foresight to allow for new inventions and products that have not been thought of yet.
The new system should be reliable and, particularly, it should be immune to electrical noise that is quite prevalent in the model train environment.
Since the existing standard for model trains is either 50 or 60 Hz for AC powered trains with DC remote control signaling or DC powered trains, it would be a tremendous advantage if remote control signaling techniques could be developed that used DC or AC remote control signaling for both AC and DC powered trains rather than more complex signaling systems such as high frequency carriers, high speed digital, touch tones, etc. A new system, certainly, should not preclude the use of more exotic signaling techniques, but it should explore the possibilities of simple AC and DC techniques first, simply because of its inherent simplicity. Considering how effectively the concept of the on-board state generator described in U.S. Pat. No. 4,914,431 can enhance the number of remote control options available for AC model railroads, it is not difficult to develop a complete system that can utilize the existing standards of either AC or DC model train environments. As long as there is a least one remote control signal available we can expand on this platform to develop a system that can effectively utilized the existing standard and allow for new systems to evolve.
With the three-rail AC train environment, this is a straight forward task using the two DC remote control signals (positive and negative) and an on-board state generator to provide additional remote control functions. However, scale two-rail trains that operate from DC applied power do not specify a remote control signal within their existing standard. In order to use the advantages of our on-board state generator, we have developed a new way to use the existing two rail DC standard to provide simple DC remote control capability. Once this is implemented, we can then develop newer systems that require more complex signaling. A major requirement of any evolutionary new system is that it allows the user to return to the standard system he now has. This will allow him to move all the way from simple block control with very limited remote control to complete command control without changing his layout.